1. Field of the Invention
The present invention relates to a battery protection circuit and a battery protection device protecting a secondary battery composed of plural cells, and a battery pack including the battery protection device.
2. Description of the Related Art
A power supply device including a secondary battery and having a function to prohibit charging of the secondary battery is known (for example, Patent Document 1).
FIG. 9 is a block diagram of the power supply device disclosed in Patent Document 1 (FIG. 10 of Patent Document 1). The power supply device monitors the cell voltages of secondary batteries 101 and 115 connected in a series using threshold voltages of MOS transistors 116 and 118. A logical sum generation circuit 120 is configured to switch off a P-channel MOS transistor 504 when the cell voltage of the secondary battery 101 is less than or equal to the threshold voltage of the P-channel MOS transistor 116, or when the cell voltage of the secondary battery 115 is less than or equal to the threshold voltage of the N-channel MOS transistor 118. With this, a FET-B 111 is switched off so that a charging current from a battery charger 108 to the secondary batteries 101 and 115 is shut off.
Further, a charge-discharge control circuit 102 is configured to monitor end voltages of the secondary batteries 101 and 115 and switch on an N-channel MOS transistor 505 when the end voltages satisfy an “overcharge condition”. With this, similarly, the FET-B 111 is switched off so that the charging current from the battery charger 108 to the secondary batteries 101 and 115 is shut off.
[Patent Document]    [Patent Document 1] Japanese Laid-open Patent Publication No. H10-225007
However, for the circuit shown in FIG. 9, the voltage between the secondary batteries 101 and 115 is directly applied to the gates of the MOS transistors 116 and 118 so that an electrostatic resistance becomes low. Therefore, in order to improve the electrostatic resistance, it is necessary to enlarge the size of the transistors such as enlarging the gate size or the like. Further, the MOS transistor 504 and the MOS transistor 505 are necessary for appropriately shutting off the FET-B 111 to prohibit charging of the secondary batteries 101 and 115, so that the size of the device becomes larger in this point as well.